The Dream of Other Earths

Twenty years ago, the only planets we knew orbited our Sun, or pulsars—the remains of stars much more massive than the Sun, so the systems weren’t at all like our Solar System.

Today, astronomers announced the discovery of an Earth-mass planet orbiting Alpha Centauri B, part of the closest star system to ours.

The planet, which bears the highly unfortunate designation Alpha Cen Bb, isn’t very Earthlike in the most important biocentric respect: it orbits the star at 1/10 the distance Mercury does, or about 4% of the Earth-Sun separation. That means its estimated surface temperature is over 1200° C—sufficiently hot to evaporate any water, boil away any atmosphere, and melt many metals. That closeness also means Alpha Cen B b orbits its host star in a mere 3.2 days, a very zippy rate.

However, the discovery is still exciting for a number of reasons. First is the proximity of the star system to us: Alpha Centauri is 4.4 light years away, a tiny distance in cosmic terms. The stars Alpha Centauri A and B are some of the brightest in the sky in the Southern Hemisphere. (Sorry, fellow Northern Hemisphere-dwellers; we can’t see them from here.) We don’t have starship technology to travel there, but we could conceivably send a robotic probe that could arrive within my lifetime, and 4.4 years isn’t a terribly long time for data to travel back to Earth. No one has such a probe in the works yet, but the mere fact of discovery of a planet might encourage investment in that direction.

Which leads to the second reason to be excited: Alpha Cen Bb is probably not the only planet orbiting Alpha Centauri B, based on what we’ve learned from other exoplanet systems (and of course our own Solar System). Models for planet formation also show that planets probably couldn’t form that close to their host stars; they likely form farther out, and migrate in. The migration mechanism isn’t fully understood (planetary scientists, feel free to yell at me if I’m getting any of this wrong), but probably interactions between planets during the formation process are what determine the final location of each world in the system. In other words, the very proximity of Alpha Cen Bb to the star is evidence in favor of the existence of other planets in the star system.

The Details

The astronomers found Alpha Cen Bb through the tiny tug it exerts on its host star. As the planet orbits, it causes the star to move just a tiny bit, which can be detected in the form of its Doppler shift: the slight variation in the color of light. The Doppler shift is familiar from sound: as an ambulance passes, its siren seems to change drastically in pitch, rising during the approach and dropping as it recedes. Similarly, as a star moves toward us, its light is shifted slightly to shorter wavelengths (blueshift), and away from us the wavelengths become slightly stretched (redshift). The more massive the planet is, the larger the resultant Doppler effect will be. Alpha Cen Bb hardly budges its host star, so the Doppler shift was really tiny—less than one meter per second, or roughly walking speed. Detecting it required four years of very careful analysis!

Alpha Centauri family portrait: B (left) and A (right).

Alpha Centauri is a star system with three stars: A, B, and Proxima. Both A and B are relatively Sunlike, with A being a bit larger and noticeably brighter, while B is a little smaller and fainter. (Proxima orbits A and B with a fairly wide swath, and is very faint; unlike the other two, you need a telescope to see it, even though it’s the closest of the three to the Solar System!) In fact, most stars in the same mass range as the Sun are in binaries or larger associations, so finding planets in those systems is a hopeful sign.

The Dream

Despite knowing about hundreds of other planets (842 confirmed discoveries as of today, not counting Alpha Cen Bb), there’s still a lot of details we haven’t learned. How common are worlds within the habitable zones of their stars? How exactly do star systems form, and what does this mean for the likelihood of life arising? With more data and more discoveries, we are gradually filling in the gaps, but exoplanetology is still a relatively new field. When I began teaching astronomy in 2006, I told students that we didn’t have any idea of the probability of finding Earthlike planets elsewhere in the Universe—not that it was likely or unlikely, but that we simply didn’t know enough.

The situation is different now. Planets are incredibly common in the galaxy; while it’s hard to find Earth-mass planets, we have hints they are everywhere. It’s only a matter of time now before we find such a world orbiting in its star’s habitable zone, and then…maybe…we’ll know what the odds are for life.